An interference observation apparatus for acquiring an interference image causes light reflected by or transmitted through an observation object and reference light to interfere with each other using an optical system of a Michelson interferometer or a Mach-Zehnder interferometer, so that an interference image of the observation object can be acquired.
The interference observation apparatuses disclosed or suggested in Non Patent Documents 1 to 5 use an optical system of the Michelson interferometer, split light output from a light source into first split light and second split light, reflect the first split light by the observation object, and combine the first split light and the second split light. Then, the interference observation apparatuses acquire an image of interference light which is generated by the combining.
Among them, the interference observation apparatuses disclosed in Non Patent Documents 1 and 2 acquire the interference image using the light source (for example, a halogen lamp or an LED (Light Emitting Diode)) which outputs incoherent light. The interference observation apparatuses disclosed or suggested in Non Patent Documents 3 and 4 acquire the interference image using the light source which outputs coherent laser light, and perform feedback control on an optical path difference between two optical paths in the Michelson interferometer on the basis of a detection result of the interference light.
The interference observation apparatus disclosed in Non Patent Document 5 acquires the interference image using a first light source which outputs incoherent light, and performs feedback control on an optical path difference between two optical paths in the Michelson interferometer on the basis of a detection result of the interference light and using a second light source which outputs coherent laser light.
The interference observation apparatuses disclosed in Non Patent Documents 7 to 11 split light output from a light source using an optical system of the Mach-Zehnder interferometer into first split light and second split light, and transmit or reflect the first split light by the observation object, and combine the first split light and the second split light. Then, the interference observation apparatuses acquire an image of interference light which is generated by the combining.
Among them, the interference observation apparatuses disclosed in Non Patent Documents 7 to 9, and 11 use the light source which outputs coherent laser light. Further, the interference observation apparatus disclosed in Non Patent Document 10 uses the light source which outputs temporally incoherent light. The interference observation apparatuses of Non Patent Documents 7 to 9 do not include a function of adjusting the optical path difference between two optical paths in the Mach-Zehnder interferometer. On the other hand, the interference observation apparatuses of Non Patent Documents 10 and 11 include the function of adjusting the optical path difference.
Here, a technique of keeping the optical path difference constant between two optical paths in the interferometer by the feedback control on the basis of the interference light detection result is called “phase lock”. A technique of changing a value of the optical path difference kept by the phase lock using the feedback control is called “phase shift”.